LED chips can generate visible or non-visible light in a specific region of the light spectrum. The light output from the LED may be in the blue, red, green, non-visible ultraviolet (UV), near-UV light, UV light, and/or light in some other region of the spectrum, depending on the material composition of the LED. When it is desired to construct an LED light source that produces a color different from the output color of the LED, it is known to convert light output from the LED having a first wavelength or wavelength range (the “primary light” or “excitation light”) to light having a second wavelength or wavelength range (the “secondary light” or “emission light”) using photoluminescence.
Photoluminescence generally involves absorbing higher energy primary light with a wavelength-conversion material such as a phosphor or mixture of phosphors. Absorption of the primary light can excite the wavelength-conversion material to a higher energy state. When the wavelength-conversion material returns to a lower energy state it emits secondary light, generally of a different and longer wavelength/wavelength range than the primary light. The wavelength/wavelength range of the secondary light depends on the type of wavelength-conversion material used. Secondary light of a desired wavelength/wavelength range may therefore be attained by proper selection of wavelength-conversion material. This process may be understood as “wavelength down conversion.” An LED combined with a wavelength-conversion structure that includes wavelength-conversion material such as phosphor to produce secondary light may be described as a “phosphor-converted LED” or “wavelength-converted LED.”
In a known configuration an LED die is positioned in a reflector cup package and volume, and is encapsulated with a polymeric encapsulating material that is directly in contact with the emitting surface of the LED. The polymer encapsulant may contain a wavelength conversion-material designed to convert primary light emitted by the LED to another wavelength, e.g., to the visible region of the electromagnetic spectrum. In another known configuration, the wavelength-conversion material may be provided in the form of a solid, self-supporting flat structure, such as a ceramic plate, a single crystal plate or a thin film structure. Such a plate may be referred to herein as a “wavelength-conversion plate.” The plate may be attached directly to the LED, e.g. by wafer bonding, sintering, gluing, etc. Such configurations, wherein a wavelength conversion material is disposed adjacent an LED chip, are referred to herein as a “chip level conversion” or “CLC” configuration. In further known configurations, a wavelength conversion material is placed remotely from the emitting surface of the LED. Such a configuration is referred to herein as a “remote conversion” configuration.
In some LED devices, a polymeric material containing fluorescent phosphor particles is used as a wavelength-conversion material to convert primary light emitted by the chip to another wavelength or wavelength range. Regardless of whether the wavelength-conversion material is used in a CLC or remote phosphor configuration, some portion of unconverted primary light emitted by the LED may be scattered by the phosphor particles and/or the polymer-air interface. To address this issue and increase the total converted light output of the device, it is known to use one or more reflective layers to reflect unconverted primary light such that it again impinges on the wavelength-conversion material.
Because titania (TiO2) highly reflects visible light, titania filled polymeric compositions are often used as primary light reflectors in light emitting diode packages. However, titania tends to absorb light at shorter wavelengths, such as in the UV region of the electromagnetic spectrum. As a result, UV light impinging on a titania-based reflector may be absorbed and lost. As a result, titania-based reflectors may be unsuitable for use as a primary light reflector in a lighting system employing LEDs that emit primary light in the UV region.
Alumina (Al2O3) containing compounds, porous polymeric films, aluminum sheets and coatings have been proposed for use as reflecting elements in UV emitting LEDs. Although alumina containing compounds reflect UV light, a high mass fraction of alumina in the compound may be needed to achieve a desired level of UV reflectance, which can increase viscosity of the compound and lead to processing issues.
Aluminum sheets and coatings are electrically conductive, making them unsuitable for applications where electrical conductivity may be undesirable, such as when a reflector is applied to the surface of an electronic circuit board. As a result, aluminum reflectors are often used in the form of one or more layers that are remote from an LED and its underlying circuitry.
Similarly, the reflectivity of porous polymeric films may decrease at wavelengths shorter than the visible region, e.g., in the ultraviolet region. Moreover, porous polymeric films are generally only useful when used in sheet form, and thus may not be a suitable replacement for a reflective coating.
Accordingly, while titania-based compositions, alumina-based compositions, aluminum sheets and coatings, and porous polymeric films may be useful for some reflector applications, they may not be ideal for applications where high UV reflectance is desired, and/or where electrical conductivity is a concern.